The present invention relates to an electron beam lithography system having a device for measuring a height of an upper surface of a specimen such as a semiconductor or mask f an electron beam in horizontal and height directions.
In the general electron beam lithography system having the height measuring means, the height of the upper surface of the specimen is usually measured by detecting a reflected light from the upper surface by a position sensor.
The Japanese Utility Model Laid-open No. 52-139768(1977), No.1-157423(1989) and No.1-167733(1989) are cited as examples of such devices.
Using exposing a pattern of a mask or a reticle on the upper surface with the apparatus stated above ,there is not a particular problem. But, in a directly exposure system, electron beam lithography system which is used in directly exposing a various patterns on the upper surface of the specimen, a serious problems arise as follows;
(1) The directly exposed patterns are usually so fine that it is needed to measure the height of the upper surface in a high accuracy in order to expose the pattern accurately.
(2) One of the light reflected from a photo-resist coated on the specimen is interfered by the other light reflected from the specimen, and the reflected light on the detector becomes so feeble that the measuring accuracy becomes low.
(3) In the directly exposure system, overlapping exposure of another pattern are repetitively performed on the pattern of the specimen which is already exposed. The reflected light is scattered by the under pattern of the specimen and it becomes impossible to detect the fully strong light.